The present invention relates to a new method for deciding the reactivity and soot index of carbon products such as granular coke and baked or burned carbon core samples, and to equipment therefor.
Everyone involved in traditional aluminum production knows full well that some of the anode material of production equipment takes part in reactions which do not benefit the production of metal. The most obvious aspect is the corrosion of the top of the anode where it comes into contact with the air. In a more concealed manner, mainly on the underside of the anode, another unfortunate process takes place. Here some of the CO.sub.2 gas from the primary reaction reacts with the carbon of the anode to form CO, carbon monoxide. According to P. J. Rhedey, Alcan International Limited, Kingston Laboratories, in "Carbon reactivity and aluminum reduction cell anodes", the reaction with air and the reaction with CO.sub.2 contribute to a significant part of consumption of the anode. The reactions with air and CO.sub.2 can, furthermore, cause the anode material to crumble, which causes operational problems with anode particles in the electrolyte, so-called sooting.
From these facts it is not difficult to understand that, for both the anode producer and the user, it is necessary to be able to predict some of the tendency of the anode to react with air and with carbon dioxide.
The reactivity of anodes can be measured in a number of ways, depending on how it is chosen to attack the problem. In general terms, regardless of the manner of attack, such a method must be at least:
selective, PA1 sufficiently sensitive, PA1 sufficiently reproducible. PA1 S=soot formed with normal test length (max. 190 min.), mg. PA1 D=sample diameter, cm. PA1 L=sample length, cm. PA1 t.sub.30 =1/2 hour.
A selective method is a method which mainly reacts to the properties of the anode which are of importance for the air/CO.sub.2 reactions under operating conditions. Satisfactory sensitivity can be defined as the ability to reveal changes in the stated properties at a level at which this would be of significance for the operation of electrolysis cells of the production equipment. Reproducibility, i.e. the distribution of results from tests on several samples from the same anode, should not be so large that it is of real significance in the assessment of the anode's user properties. This is the same as saying that, regardless of how many times a piece of carbon is tested, the conclusion with regard to its quality must remain the same.
In the method of analysis which forms the object of the present invention, instead of scaling down operating conditions in electrolysis cells to manageable laboratory size, emphasis is placed on developing a reproducible test which can be employed to carry out quality grading of anodes. In addition, the point of departure for the grading is the hypothesis that the lowest possible reactivity and sooting are desirable.
The tendency of an anode to react, i.e. how rapidly the reaction takes place, has been designated "reactivity". Thus, there are two types of reactivity, namely CO.sub.2 reactivity and air reactivity. Both reactions lead to a gasification of a sample, i.e. it loses weight. By registering this loss of weight, it is possible to obtain a measurement of the reactivity.
Sooting is caused in this connection by the reactivity being different in the aggregate and the binding agent of the anode material. If the binding coke, for example, reacts least, this will cause the particles in the aggregate to be undermined and gradually to fall out during the test (and under operating conditions). After the sample has been exposed to the reactant gas over a certain period of time, it is brushed and all the loose material is collected and weighed. This is thus soot in the context of the analysis. It has been chosen to express the level of sooting as the ratio of the weight of the soot and the overall loss in weight, where the overall loss of weight is the total weight of the gasified part and the part brushed off.
The reactivities are determined by means of a regression analysis for the last 30 minutes of reaction time. The following formulae apply. ##EQU1## G=gasified (weight loss) during the whole test (max. 190 min.), mg. G.sub.30 =gasified (weight loss) during the last 30 min., mg.
In the test methods for deciding air and CO.sub.2 reactivity which have been known up to now, at least two independent tests must be carried out, one to decide the air reactivity and another to decide the CO.sub.2 reactivity.